The only icing or frosting that I can think of that would give a smooth surface would be a Royal Icing or if you wanted Chocolate a Ganache. The Royal would set up firm the ganache would be a bit softer. Royal icing is a mixture of egg whites and powdered sugar. I do not have an exact recipe with me but a quick search would bring up several.
When the ganache is first made it is thin so it can be used as a glaze. I would make the ganache, allow it to set for a bit so it is just a bit thicker then dip what ever you are going to dip. Allow the ganache to set even further then use as a frosting. Whipping the ganache once it has firmed up a bit you will have a spreadable frosting.